I ought to cut the ones on the XC HT down to about 660mm, 640 or less would be better uphill for me I reckon, but I know is a bit too narrow when things get more challenging. I'm not I'd want (much) less than 685mm if I SS it again though

The FS wants to go wider I reckon, to about 710/711mm. It'd benefit when things get really challenging, though as I already find myself putting my hands inboard a bit on smoother climbs, I reckon more techincal climbs that bit harder with my arms forcably splayed* out further over the wide bars

600mm on the commuter (With stubby bar ends) often feels too wide, I very often ride with my hands over the shifters/brake levers)

On the 'play' bike (a sort of 4X/DJ bike) 685mm feels fine I think because the bars are so much lower (and the saddle is right out of the way

I know that when I played with a higher rise stem on the FS (which made my drops/jumps more consitent(ly right)) the bars felt too narrow, I just didn't feel I had the leverage side-to-side with the extra height. I think the height of the bars makes a big difference as to how wide feels right. The higher the bars, the wider the bars seem to need to be to retain the same control/leverage over the bike

600mm on my hardtail.
But today I put new cables on my East Peak and took it for it's first ride in over 3 years. Well I thought as I rode along I reckon these bars are a much better width, think I'll look at changing the bars on the hardtail. So as soon as I got home I measured them to see what I should be looking for. Looks like I should be changing to 600mm.

In my defence I think the real difference was the increased sweep of the bars on the East Peak.

i'm 5ft 8 with shortish arms, 685mm with 70mm stem on the giant trance and 710mm with 50mm stem on the marin quake DH bike.
Was wondering today whether to go for wider bars but i seem to do alright on both bikes and the 685mm easton ea70 bars did feel bloody fantastic (angles as well as width) after the skinny ass things i had before.

I think a part of this is fashion for wide bars at the moment and a part of it is about the width of your shoulders. me I am fairly narrow shouldered so run 660 on the solo and 685 on the tandem where the greater leverage is a help. I find the 660 more comfortable and will go back to that for touring teh tandem

Started riding back in 1994 and used what my bike came with, which back then as was the norm, was 560mm flat bars with bar ends fitted... So not very wide! Still, quite a bit wider than the bars on my mates bike at the time, which had been trimmed to sub 500mm when he inherited it!

Use 685mm or 710mm wide bars now, have tried riding bars wider but my shoulders just ain't that wide so it's more of a hindrance than a help.